Ball bearing

Abstract

Provided is a ball bearing that used in liquefied gas and comprises: a pair of bearing rings composed of an inner ring and an outer ring; a plurality of balls interposed between the raceway surfaces of the pair of bearing rings; and a retainer for holding the plurality of balls between the pair of bearing rings. The retainer is an inner ring–guided retainer, wherein the retainer rotates as a result of causing the inner radial surface of the retainer and the outer radial surface of the inner ring to slidably contact each other with a predetermined radial gap therebetween, and the guide gap between the outer radial surface of the inner ring and the inner radial surface of the retainer is 3-10% of the diameter of a ball.

Description

ball bearing

[0001] The present invention relates to a ball bearing for use at extremely low temperatures.

[0002] There are rolling bearings for use in cryogenic environments, such as bearings used in submerged pumps that transport liquefied gases (e.g., LNG, LN, LNH, and LH) at cryogenic temperatures (Patent Documents 1 and 2). Because rolling bearings used in such liquefied gas pumps are used while immersed in cryogenic liquefied gas, they are required to have wear resistance and low-temperature toughness at cryogenic temperatures.

[0003] In the technology of Patent Document 1, the linear expansion coefficient of the rolling element material is set to 70 to 105% of the linear expansion coefficient of the raceway material, and the cage material contains PTFE, a fibrous reinforcing material, and a solid lubricant, thereby improving wear resistance.

[0004] Patent Document 2 specifies an optimal range for the gap ratio C = (rolling element diameter × guide gap) / (inner ring outer diameter × radial internal gap) for deep groove ball bearings. Here, the guide gap is the gap width between the outer diameter surface of the inner ring and the inner diameter surface of the cage. By specifying the gap ratio C, the bearing can withstand long-term use, with no deterioration in wear resistance or lubrication over time, and the rotation state can be stabilized.

[0005] International Publication No. 2015-053348 Japanese Patent Application Laid-Open No. 2017-150593

[0006] Rolling bearings used at extremely low temperatures experience dimensional shrinkage in their bearing components (race rings, rolling elements, and cages). Because the raceways, rolling elements, and cages are made of different materials, they shrink by different amounts at low temperatures, and each has a different linear expansion coefficient. Therefore, optimal design of the bearing components is necessary to ensure smooth rotation at low temperatures. Of these, the cage is the component that experiences the greatest difference in dimensional change and is therefore the most affected. Therefore, optimal design of the cage's guide clearance (radial clearance between the cage and the guiding raceway) and guide type is important to ensure proper function at extremely low temperatures.

[0007] In view of the above circumstances, an object of the present invention is to provide a ball bearing that can rotate well even at extremely low temperatures where dimensional shrinkage of bearing components occurs.

[0008] The ball bearing of the present invention is a ball bearing for use in liquefied gas, comprising a pair of raceways consisting of an inner ring and an outer ring, a plurality of balls interposed between the raceway surfaces of the pair of raceways, and a cage that holds the plurality of balls between the pair of raceways, wherein the cage is an inner ring guide that rotates by causing the inner diameter surface of the cage and the outer diameter surface of the inner ring to slide against each other with a predetermined radial gap between them, and the radial gap (guide gap) formed between the outer diameter surface of the inner ring and the inner diameter surface of the cage is 3% to 10% of the ball diameter, which is the diameter of the balls.

[0009] The ball bearing of the present invention defines the guide clearance of the cage (the radial clearance formed between the outer diameter surface of the inner ring and the inner diameter surface of the cage), which experiences a large difference in dimensional change at low temperatures, and the guide format of the cage. That is, the cage rotates by allowing the inner diameter surface of the cage and the outer diameter surface of the inner ring to slide against each other across a defined gap, a so-called inner ring guide, which allows the cage to rotate well even at extremely low temperatures where dimensional shrinkage occurs. In the case of an outer ring guide cage, the guide clearance expands due to dimensional shrinkage, and proper guidance is not possible. That is, with an outer ring guide cage, if it is designed to have an appropriate guide clearance at low temperatures, it will be in an expanded state at room temperature, so will come into contact with the inner diameter of the outer ring and will not be able to be assembled. If the dimensions are such that assembly is possible, it will contract at low temperatures, causing the guide clearance to expand. This can cause the guiding to become unstable (this is particularly true for angular contact ball bearings with an outer ring counter-shaped configuration), or rolling element guide will be dominant.

[0010] Furthermore, if the cage is guided by the inner ring, if the guide clearance / ball diameter is too small, the outer diameter of the inner ring and the inner diameter of the cage will come into contact at low temperatures, resulting in poor rotation. Also, if the guide clearance / ball diameter is too large, the guide clearance will exceed the pocket clearance, resulting in rolling element guidance. Therefore, by setting the guide clearance to 3 to 10% of the ball diameter, an optimal guide clearance is achieved that prevents contact with the inner ring outer diameter at low temperatures. This ensures inner ring guidance while maintaining the guide clearance between the inner ring outer diameter and the inner diameter of the cage, allowing for normal operation even at extremely low temperatures.

[0011] In the above configuration, a tapered relief portion that expands in diameter from the inside to the outside in the axial direction can be provided on the axially outer side of the cage inner diameter portion. In this case, the inclination angle of the relief portion in the axial direction is preferably 10° to 45°. This makes it easier for liquefied gas to flow as a lubricant between the outer diameter of the inner ring and the cage inner diameter, forming an oil film and improving the lubrication state.

[0012] In the above configuration, the cage can be made of a special resin material containing polytetrafluoroethylene (PTFE) and glass fiber. Ball bearings used at extremely low temperatures are immersed in liquefied gases such as LNG, so lubricants such as oil and grease cannot be used. Liquefied gas itself is used as a lubricant, but liquefied gas has low viscosity and poor lubricant performance. Using a special self-lubricating resin made of PTFE and glass fiber as the cage material provides wear resistance in extremely low-temperature environments where lubricants such as oil and grease cannot be used, thereby extending the bearing's life in liquefied gas. In this case, the raceway rings can be made of SUS440C and the rolling elements can be made of ceramic.

[0013] As described above, according to the present invention, it is possible to obtain a ball bearing that can rotate well even at extremely low temperatures where dimensional shrinkage of bearing components occurs.

[0014] It is a cross-sectional view of a main part of a ball bearing according to an embodiment. It is a cross-sectional view of a cage constituting the ball bearing. It is an enlarged view of a main part of Fig. 1. It is a schematic configuration diagram of a submerged pump for liquefied natural gas, explaining a usage state of an embodiment.

[0015] 1 shows a longitudinal cross section of a ball bearing according to one embodiment of the present invention. This ball bearing 1 is equipped with a pair of raceways consisting of an inner ring 2 having an inner raceway surface 2b on its outer diameter surface 2a and an outer ring 3 having an outer raceway surface 3b on its inner diameter surface 3a, a plurality of balls 4 as rolling elements arranged to roll freely between the raceways 2b, 3b, and an annular cage 5 arranged between the inner ring 2 and the outer ring 3.

[0016] The ball bearing 1 of this embodiment is a so-called inner ring guide bearing in which the inner diameter surface 5a of the cage 5 and the outer diameter surface 2a of the inner ring 2 are in sliding contact with each other across a specified gap, thereby guiding the cage 5. For this reason, the outer diameter surface 2a of the inner ring 2, excluding the inner raceway surface 2b, serves as a cage guide surface 6 (hereinafter referred to as the guide surface 6) that is in sliding contact with the cage 5.

[0017] The cage 5 is inner ring guided. In the case of a so-called outer ring guided cage, dimensional shrinkage expands the guide clearance (the gap between the outer diameter surface 2 a of the inner ring 2 and the inner diameter surface 5 a of the cage 5), resulting in improper guidance. In other words, if the outer ring guided cage has an appropriate guide clearance at low temperatures, the cage will be in an expanded state at room temperature, coming into contact with the inner diameter of the outer ring and making assembly impossible. If the dimensions are such that assembly is possible, the cage will contract at low temperatures, expanding the guide clearance. This can cause unstable guidance (particularly in the case of angular contact ball bearings with an outer ring counter-shaped), or rolling element guidance may become dominant. Therefore, by using inner ring guided cages as in this embodiment, good rotation is possible even at extremely low temperatures, where dimensional shrinkage occurs.

[0018] The raceways, that is, the inner ring 2 and the outer ring 3, are preferably made of stainless steel. Examples of martensitic stainless steel include SUS403, SUS420, and SUS440C, with SUS440C being particularly preferred.

[0019] The rolling elements 3 are preferably made of stainless steel or ceramics. The type of ceramic is not particularly limited, and ceramics based on silicon nitride, zirconia, silicon carbide, and alumina can be prepared. For example, rolling elements made of silicon nitride ceramics are preferred because they are particularly hard and have excellent wear resistance.

[0020] The cage 5 is made of a resin material primarily composed of polytetrafluoroethylene (PTFE), polypropylene, polystyrene, acrylonitrile styrene, acrylonitrile-butadiene-styrene plastic, polytrifluorochloroethylene, polycarbonate, polymethyl methacrylate, polyamide 6, polyamide 66, polysulfone, polyphenylene oxide, phenolic resin, epoxy resin, unsaturated polyester resin, urea resin, or melamine resin. For particularly good lubricity, a resin made of polytetrafluoroethylene (e.g., BEAREE FL3000 manufactured by NTN Corporation) is used. This resin material transfers solid lubricant to the rolling element surface even at extremely low temperatures, providing good and stable solid lubrication. Furthermore, using a special resin material containing glass fiber in addition to PTFE is particularly preferable because it provides wear resistance in addition to lubricity.

[0021] Because the cage 5 experiences large dimensional changes at low temperatures, it is necessary to specify the guide clearance in the inner ring guide to ensure smooth rotation at low temperatures. As shown in Figure 1, the guide clearance Sr between the outer diameter surface 2a of the inner ring 2 and the inner diameter surface 5a of the cage 5 at room temperature is set to 3% to 10% of the ball diameter Da, which is the diameter of the balls. That is, as shown in Figure 1, if the inner diameter of the cage is Hd and the outer diameter of the inner ring is d1, then Sr = Hd - d1 = 0.03 Da to 0.10 Da. Note that room temperature refers to an ordinary temperature without any particular cooling or heating, and specifically refers to a predetermined temperature in the range of 5°C to 35°C (in accordance with JIS 8703).

[0022] When the cage 5 is guided by the inner ring, if the guide clearance / ball diameter (Sr / Da) is too small, the outer diameter of the inner ring and the inner diameter of the cage will come into contact at low temperatures, resulting in poor rotation. Furthermore, if the guide clearance / ball diameter (Sr / Da) is too large, the guide clearance Sr will exceed the pocket clearance, resulting in rolling element guidance. Therefore, by setting the guide clearance Sr to 3 to 10% of the ball diameter Da, an optimal guide clearance Sr will be achieved that prevents contact with the outer diameter of the inner ring at low temperatures. This ensures inner ring guidance while maintaining the guide clearance Sr between the outer diameter of the inner ring and the inner diameter of the cage, enabling normal operation even at extremely low temperatures.

[0023] As shown in FIG. 2 , a tapered relief portion 7, which expands in diameter from the inside to the outside in the axial direction, is provided on both axially outer sides of the cage inner diameter portion (i.e., both axially outer edge portions of the portion where the inner diameter surface 5a of the cage 5 is present). The inclination angle TH of the relief portion 7, i.e., the inclination angle TH of the relief portion 7 with respect to the cylindrical surface formed by the inner diameter surface 5a of the cage 5, is preferably 10° to 45°. As shown in FIG. 3 , a chamfered portion 8 is formed on each of both axially outer sides of the outer diameter surface 2a of the inner ring 2. If the axial length from the axially inner end 8a of the chamfered portion 8 to the axially inner end 7a of the relief portion 7 of the cage 5 is A, the relationship between A and the axial length HBA of the relief portion 7 (see FIG. 2 ) can be, for example, HBA = A > 0. The provision of such relief portions 7 facilitates the flow of liquefied gas as a lubricant between the inner ring outer diameter portion 2a and the cage inner diameter portion 5a, forming an oil film and improving lubrication.

[0024] The ball bearing of this embodiment can rotate well even at extremely low temperatures, which causes dimensional shrinkage of bearing components.

[0025] The ball bearing of the present invention is used in a submerged pump that transfers liquefied gases such as LNG, LN2, LNH3, and LH2 at extremely low temperatures. As shown in Figure 4, a submerged pump for liquefied natural gas (LNG) is designed to achieve airtightness within a pot (pressure vessel) 8 by immersing the entire pump in liquid, and the pump shaft 9 is integrally and coaxially connected to a motor shaft 10.

[0026] The pot 8 has an LNG suction port 11 that opens outward and a discharge port 12 that communicates with external piping (not shown). A motor 13 installed in the pot 8 has a motor shaft 10 that rotates using an external power source, and the upper and lower sides of the motor shaft 10 are supported by ball bearings A of the embodiment shown in Fig. 1. A multi-stage impeller 14 is attached to a pump shaft 9 that rotates integrally with the motor shaft 10.

[0027] In the illustrated pump, the flow path within the device is such that LNG flowing into the pot 8 from the suction port 11 flows downward along the inner surface of the pot 8 by the impeller 14, which rotates integrally with the pump shaft 9 by the driven motor 13, is sucked in from the lowest stage of the multi-stage impeller 14, and flows from piping 16 inside the cylindrical inner wall 15 arranged around the impeller 14 to the discharge port 12, but some of the LNG flows from another piping 17 inside the cylindrical inner wall 15 through the motor 13 as a lubricating liquid, lubricating and cooling the ball bearings A, joins the downward flow along the inner surface of the pot 8, and is sucked in again from the tip of the multi-stage impeller 14.

[0028] Although the present invention has been described above in terms of an embodiment, it is not limited to the above embodiment and various modifications are possible, and the materials of the outer ring, inner ring, cage, and balls are not limited to those in the embodiment. The cryogenic environment ball bearing of the present invention may be used as a ball bearing for a liquefied gas pump, or as a ball bearing for supporting or driving a satellite antenna.

[0029] A ball bearing, model number 7204 in Table 1 and model number 7305 in Table 2, was assembled using an inner and outer ring made of martensitic stainless steel (SUS440C), balls made of silicon nitride ceramics as rolling elements, and a ring-shaped cage, model number 7204 (Table 1) and model number 7305 (Table 2), manufactured by NTN Precision Plastics Corporation, which is made primarily of polytetrafluoroethylene (PTFE).

[0030] Table 1 shows the results of judging the operating performance of ball bearings at cryogenic temperatures (-190°C) for model 7204 ball bearings with a ball diameter of 7.9375 mm, with the guide clearance / Da (ball diameter) ratio at room temperature (approximately 20°C) varied in five stages: 0.02, 0.03, 0.07, 0.10, and 0.13. Operating performance was evaluated by measuring the guide clearance and pocket clearance at low temperatures (indicated by ◯, △, and × in the table).

[0031] As a result, when the guide clearance / Da (ball diameter) ratio was 0.07, the guide clearance was appropriately smaller than the pocket clearance, resulting in good operation (determined as ◯ in Table 1). Also, when the guide clearance / Da (ball diameter) ratio was 0.10, the guide clearance was smaller than the pocket clearance, resulting in the second best operation after the guide clearance / Da ratio of 0.07 (determined as △ in Table 1). On the other hand, when the guide clearance / Da (ball diameter) ratio was 0.02, 0.03, or 0.13, the guide clearance was larger than the pocket clearance or the inner diameter of the cage came into contact with the outer diameter of the inner ring, resulting in poor operation (determined as x in Table 1).

[0032] Table 2 shows the results of judging the operating performance of ball bearings at cryogenic temperatures (-162°C) for model 7305 ball bearings with a ball diameter of 11.1125 mm, with the guide clearance / Da (ball diameter) ratio at room temperature (approximately 20°C) varied in five stages: 0.02, 0.03, 0.07, 0.10, and 0.13. Operating performance was evaluated by measuring the guide clearance and pocket clearance at low temperatures.

[0033] As a result, when the guide clearance / Da (ball diameter) ratio was 0.07, the guide clearance was appropriately smaller than the pocket clearance, resulting in good operation (determined as ◯ in Table 2). Also, when the guide clearance / Da (ball diameter) ratio was 0.03, the guide clearance was smaller than the pocket clearance, resulting in the second best operation after the guide clearance / Da ratio of 0.07 (determined as △ in Table 2). On the other hand, when the guide clearance / Da (ball diameter) ratio was 0.02, 0.10, or 0.13, the guide clearance was larger than the pocket clearance or the inner diameter of the cage came into contact with the outer diameter of the inner ring, resulting in poor operation (determined as x in Table 2).

[0034] Table 3 shows the results of judging the operating performance of ball bearings at cryogenic temperatures (-50°C) for model 7007 ball bearings with a ball diameter of 7.9375 mm, with the guide clearance / Da (ball diameter) ratio at room temperature (approximately 20°C) varied in five stages: 0.02, 0.03, 0.07, 0.10, and 0.13. Operating performance was evaluated by measuring the guide clearance and pocket clearance at low temperatures.

[0035] As a result, when the guide clearance / Da (ball diameter) ratio was 0.07, the guide clearance was appropriately smaller than the pocket clearance, resulting in good operation (determined as ◯ in Table 3). Also, when the guide clearance / Da (ball diameter) ratio was 0.10, the guide clearance was smaller than the pocket clearance, resulting in the second best operation after the guide clearance / Da ratio of 0.07 (determined as △ in Table 3). On the other hand, when the guide clearance / Da (ball diameter) ratio was 0.02, 0.03, or 0.13, the guide clearance was larger than the pocket clearance or the inner diameter of the cage came into contact with the outer diameter of the inner ring, resulting in poor operation (determined as x in Table 3).

[0036] Table 4 shows the results of judging the operating performance of ball bearings at cryogenic temperatures (-250°C) for model 7010 ball bearings with a ball diameter of 8.7313 mm, with the guide clearance / Da (ball diameter) ratio at room temperature (approximately 20°C) varied in five stages: 0.02, 0.03, 0.07, 0.10, and 0.13. Operating performance was evaluated by measuring the guide clearance and pocket clearance at low temperatures.

[0037] As a result, when the guide clearance / Da (ball diameter) ratio was 0.07, the guide clearance was appropriately smaller than the pocket clearance, resulting in good operation (determined as "good" in Table 4). Also, when the guide clearance / Da (ball diameter) ratio was 0.10, the guide clearance was smaller than the pocket clearance, resulting in good operation similar to when the guide clearance / Da ratio was 0.07 (determined as "good" in Table 4). On the other hand, when the guide clearance / Da (ball diameter) ratio was 0.02, 0.03, or 0.13, the guide clearance was larger than the pocket clearance or the inner diameter of the cage and the outer diameter of the inner ring came into contact, resulting in poor operation (determined as "poor" in Table 4).

[0038] From the results in Tables 1 to 4, when the guide gap / Da was 0.07, good results were obtained in all environments. Furthermore, when the guide gap / Da was 0.10, generally good results were obtained, but there were also cases where it was not good (Table 2), so it can be said that it operates well depending on the conditions. Furthermore, when the guide gap / Da was further increased to 0.13, it did not operate well in all cases, so it was found that 0.10 is an appropriate upper limit for the guide gap / Da.

[0039] On the other hand, when the guide clearance / Da is 0.03, there are cases where good results are obtained, but not always (Table 2), and it can be said that it operates well depending on the conditions. Furthermore, when the guide clearance / Da is further reduced to 0.02, it did not operate well in all cases, so it was found that 0.03 is an appropriate lower limit for the guide clearance / Da.

[0040] The ball bearing of the present invention can be used for liquefied gas pumps and for supporting and driving devices for satellite antennas.

[0041] REFERENCE SIGNS LIST 1 Bearing 2 Inner ring 2a Outer diameter surface 3 Outer ring 4 Ball 5 Cage 5a Inner diameter surface 7 Relief portion Sr Guide clearance

Claims

1. A ball bearing for use in liquefied gas, comprising a pair of raceways consisting of an inner and outer ring, a plurality of balls interposed between the raceway surfaces of the pair of raceways, and a cage that holds the plurality of balls between the pair of raceways, wherein the cage is an inner ring guide that is guided by sliding contact between the inner diameter surface of the cage and the outer diameter surface of the inner ring, and the guide clearance between the outer diameter surface of the inner ring and the inner diameter surface of the cage is 3% to 10% of the ball diameter, which is the diameter of the balls.

2. A ball bearing according to claim 1, characterized in that a tapered relief portion is provided on the axially outer side of the inner diameter portion of the cage, the diameter of which increases from the inside to the outside in the axial direction.

3. A ball bearing according to claim 2, characterized in that the angle of inclination of said recess in the axial direction is 10° to 45°.

Images